1. Field of the Invention
The present invention relates generally to an orthogonal frequency division multiplexing transmission technology and, more particularly, to an orthogonal frequency division multiplexing transmission apparatus and method that are implemented to minimize the transmission delay of a frame when the frame received from an upper layer is transmitted.
2. Description of the Prior Art
Recently, Internet telephones have been popularized, and several wireless Internet telephones using Wireless Local Area Network (WLAN) technologies based on the IEEE 802.11 standard have been commercialized.
One of the factors that must be considered to apply WLAN technologies to voice communication is a transmission delay problem. If the transmission delay between transmitting and receiving parties in voice communication is long, an echo may be generated and degrade the quality of communication. Accordingly, efforts to minimize transmission delay as much as possible must be made.
An OFDM scheme, that is, one of the WLAN technologies, is a digital modulation scheme for improving a transmission rate per unit bandwidth and preventing multi-path interference, and is a multi-carrier modulation scheme using a plurality of carriers that are orthogonal to one another. Furthermore, the OFDM scheme allows many carriers to be multiplexed compared to a common Frequency Division Multiplexing (FDM) scheme, so that the OFDM scheme has high frequency use efficiency and a transmission rate per unit bandwidth can be increased when the number of carriers increases.
A typical implementation example in which a transmitter is implemented using such an OFDM scheme is shown in FIG. 1. Generally, in an OFDM transmitter, data to be transmitted are transmitted to a scrambler 101 with a transmission rate set thereon. The data scrambled in the scrambler 101 are input to an interleaver 103 through a convolution encoder 102. The data are interleaved with each other in the interleaver 103. A constellation mapper 104 maps the interleaved data according to a transmission data rate, and then transmits the mapped data to a Radio Frequency (RF) converter 108 through an inverse fast Fourier transformer 105, a guard-interval inserter 106 and a symbol wave-shaping filter 107.
FIG. 2 is a schematic view illustrating the format of a frame that is transmitted from the OFDM transmitter of FIG. 1. When the frame shown in FIG. 2 is output through the OFDM transmitter of FIG. 1, a preamble 21 is output as ten short training symbols during the first 8 μs (microseconds) and then as two long training symbols during a subsequent 8 μs and a signal field 22 and a data field 23 are output as OFDM symbols on a 4 μs (i.e., a transmission interval basis) basis, as shown in FIG. 3.
In order to implement such a transmitter to have minimal transmission delay, it is required that the training symbols shown in FIG. 3 be output from the transmitter immediately after a data transmission request is generated. Although not particularly defined in the IEEE 802.11 standard, to fulfill the above-described condition, the transmitter must be constructed so that the training symbols corresponding to the preamble 21 are previously stored in a training symbol storage and generation unit 150, the training symbols are output from the training symbol storage and generation unit 150 immediately after a data transmission request is generated, and OFDM symbols that correspond to the signal field 22 and the data field 23 are output immediately after the output of the training symbols is completed, as shown in FIG. 4.
Conventionally, there is no particular requirement for how the OFDM transmitter is implemented so as to have minimal transmission delay. The construction of the OFDM transmitter shown in FIG. 4 may have a different purpose, and can be easily contrived by any person having a basic knowledge of electronic or communication engineering on the basis of the IEEE 802.11 standard.
An example of such construction is disclosed in U.S. Pat Appl. Pub No. 2004/0160892 A1 (published on Aug. 19, 2004) entitled “Orthogonal Frequency Division Multiplexing Transmitter System and VLSI Implementation Thereof.” The system of the preceding patent is constructed in such a way that a time-domain preamble is previously stored in a memory, and is then transmitted without passing through other function blocks immediately after a data transmission request is generated. However, the preceding patent discloses only that very small transmission delay is achieved by causing the preamble to be directly output without passing through other function blocks, but does not disclose the time points at which a signal field and a data field will be output in consideration of the data processing of other function blocks. In other words, the preceding patent does not describe the time points at which the data of the signal field and the data field are requested from an upper layer while the training symbols of the preamble are output, in consideration of the data processing time of the function blocks. Accordingly, the implementation of an OFDM transmitter having minimal transmission delay, which the present invention attempts to achieve, has not been disclosed.
Meanwhile, an OFDM modulation/demodulation apparatus and method, which support a variable data rate in a WLAN system, are disclosed in Korean Pat No. 0375824 issued on Feb. 28, 2003. This patent provides the OFDM modulation/demodulation apparatus and method that can support a variable data rate prescribed in the IEEE 802.11a standard. In particular, the apparatus and method allow modulation and demodulation based on a data rate of 6 Mbps to 54 Mbps to be processed in a single device. However, this OFDM modulation/demodulation apparatus does not teach a method of implementing a transmission apparatus having minimal transmission delay.